Determination of the absolute configurations of gamma-rubromycin and related spiro compounds by quantum chemical CD calculations

The absolute configurations of the spiro compounds gamma- and beta-rubromyc in (2 and 3), natural antibiotics derived from actinomycetes, have been elu cidated as (S) by quantum chemical calculations of their chiroptical proper ties and comparisons with the respective experimental CD spectra. Surprisin gly, their spiro centers show an absolute configuration opposite to that of heliquinomycin (1), the only related spiro compound for which the configur ation has hitherto been assigned. By further quantum chemical CD calculatio ns, we were likewise able to confirm the absolute stereostructure of heliqu inomycin (1), previously assigned by X-ray structural and degradative studi es, thus providing independent corroboration of our results for gamma- and beta-rubromycin (2 and 3). Accordingly, it has emerged that diverse actinom ycetes can generate different acetal configurations.